Network Address Translation (NAT) traversal is a challenge in computer networking that has become a ubiquitous factor that must be taken into consideration when creating new protocols, technologies and services. In current networks, NAT is deployed as a means of security, address-space and network topology abstraction in addition to the originally intended purpose of extending diminishing IPv4 address space.
Because of the variety in applications of NAT as a technology, differing requirements has caused great divergence in how a NAT-function is implemented on a given network device. For example, some NAT implementations first and foremost consider security as primary objective, while others consider scalability as a primary objective. Regardless of the type of NAT implementation it remains a challenge for an application to operate transparently whether or not a NAT-device is present in the network transport path or not. This challenge is magnified in cases where multiple different types of NAT implementations must be considered concurrently and in combination. Such deployments are common in current networks and present a very real difficulty when trying to provide transparent connectivity for an application.
The common types of NAT-implementation are the following:                a. Endpoint Independent (also know as Full Cone), establishes a translation entry between the inside private address and the outside public address and allows any incoming connection from the outside to be established with to the private address        b. Address Dependent (also known as Restricted Cone), establishes a translation entry between the inside private address and the outside public address and only allows incoming connections from the outside originating from the address the original flow was using as the destination address.        c. Address and Port Dependent (also known as Port-Restricted Cone), establishes a translation entry between the inside private address and the outside public address and only allows incoming connections from the outside originating from the address and upper layer protocol port the original flow was using as the destination address and port.        d. Symmetric, establishes a translation entry between the inside private address and the outside public address where the outside upper layer protocol port is uniquely assigned to every Source Address/Port and Destination Address/Port flow that creates the translation entry in the NAT. Any incoming connection not exactly matching the outside Source Address/Port and Destination Address/Port is disallowed.        
NAT Traversal through an Endpoint Independent NAT does not require any specific actions, but for the other types of NAT there are restrictions that can be handled in a variety of ways, but there is no single approach that can be used to ensure traversal through all the types of restricted NAT-implementations (Address Dependent, Address and Port Dependent, and Symmetric).